1. Field of the Invention
This invention relates to electrical circuits, and more particularly, to an electrical circuit which is capable of acting as an interface between other circuits which may have different signal and/or power supply levels.
2. Description of the Prior Art
In certain situations, it may be necessary to provide coupling of operation between two circuits which have different signal levels and/or different power supply levels. For example, one circuit may be based on small-swing ECL-like (Emitter Coupled Logic) signals referenced to a positive voltage V.sub.cc, and that circuit must interface with a circuit providing TTL-like (Transistor-Transistor Logic) signals which have large signal swings that may be referenced to ground. The circuits which provide such interface function are known as translator circuits.
In such translator circuits, it is most important that the output signal therefrom be accurately tied to the input threshold of the following circuit, and that sufficiently high gain be provided.
A commonly used ECL to TTL translator based on bipolar technology is shown in FIG. 1. In the direct current, balanced situation, the voltage at the respective bases of transistor Q1 and Q2 are the same, and nodes 1 and 2 are at the same threshold voltage, i.e., V.sub.BE above ground (node 1 being measured through transistor Q3 or transistor Q4, and node 2 being measured through transistor Q5). In such situation, the current through resistor R1 and transistor Q3 on the one hand, and the current through resistor R2 and transistor Q4 on the other hand, are substantially the same.
When a differential input signal (in the form of inverse signals of small swing) is applied to the bases of respective transistors Q1, Q2, this results in the current flowing through resistor R1 being different from the current flowing through resistor R2. However, since the bases of the respective transistors Q3 and Q4 are at the same voltage level, i.e., V.sub.BE above ground, substantially identical currents flow through transistors Q3 and Q4. A difference in the current flowing through resistor R2 and transistor Q4 will either inject an excess current into the base of transistor Q5 to saturate transistor Q5, or will pull current from the base of transistor Q5 to turn off transistor Q5, depending on the levels of the signals applied to the respective bases of transistors Q1 and Q2. Through the respective saturation and turning off the trnsistor Q5, a high voltage swing will be obtained at the output lead O/P.
The gallium arsenide-based field effect transistor circuit of FIG. 2 is laid out in a manner generally similar to that of the bipolar circuit of FIG. 1. However, in the case of the circuit of FIG. 2, because of the low threshold voltage of the transistor Q5, fully on transistor Q4 may not be able to fully turn off transistor Q5. In order to ensure that transistor Q5 is off when needed, level shifting is required (FIG. 3), in the form of a diode D1 connected between the source of transistor Q5 and ground along with bleeder transistor Q6 which connects to the voltage supply terminal and to the source of transistor Q5 to insure proper level shifting by the diode D1. While the inclusion of such diode D1 raises the threshold voltage of the following stage including transistor Q5 to ensure that a fully on transistor Q4 will turn the transistor Q5 off, it will be seen that the voltage level at node 1 is different from the voltage level at node 2, at threshold of the stage including Q5 because of the extra diode drop provided by the diode D1. That is, in steady state operation, at threshold voltage of the stage including transistor Q5, node 1 of FIG. 3 is at a voltage of V.sub.GS above ground (either through transistor Q3 or transistor Q4), while node 2 is at a threshold voltage V.sub.GS (transistor Q5) and one diode drop (.phi.) above ground. Thus, the input threshold is no longer balanced.
In "OPERATIONAL AMPLIFIERS", Bipolar and MOS Analog Integrated Circuit Design by Alan B. Grebene (published 1984 by John Wiley & Sons, Inc.) a bipolar circuit of general interest is disclosed. However, this circuit does not address the problems described above regard to field effect transistors, and is in fact a linear device implemented, as set forth above, in bipolar technology rather than a digital circuit implemented in field effect transistor technology.